Polycarbonate (PC) is one of the most important engineering plastics which have high thermal stability and impact resistance. In order to improve its durability and chemical resistance blends of PC and various thermoplastics were developed and successfully used in wide range of applications. Most useful PC-based blends include PC/Acrylonitrile butadiene styrene (ABS), PC/Polybutylene terephthalate (PBT) and PC/Polyethylene terephthalate (PET) blends. Excellent mechanical properties and chemical resistance of these blends enabled their use in number of fields including electrical appliances and automotive applications.
There were a number attempts to introduce bio-based content in PC blends. Most of these attempts were aimed on poly(carbonate) based poly(lactic acid) (PLA) blends. These blends have intrinsic poor interphase compatibility resulting in poor impact strength and low heat deflection temperature. Therefore, research was mostly aimed on compatibilization of PC/PLA blends and improving mechanical properties via introduction of various additives to overcome brittleness and low heat resistance.
United States Patent Publication No. 2014/0235740 discloses a composition containing PLA and PC where talc was added to obtain heat resistant blend. The talc in the range of 2 to 9 wt. % improved heat resistance of the blend while optional added impact modifier increased the impact strength of the blend. The processing was done in the range of temperatures of 200-220° C.
In United States Patent Publication No. 2014/0200295 this composition was further modified to increase its flame retardant properties.
United States Patent Publication No. 2013/0137804 A1 describes a copolymer of polycarbonate used for blending with poly(lactic acid) to obtain a blend with superior impact properties. Polycarbonate used as example in this application had weight average molecular weight in range 27000-38000.
U.S. Pat. No. 5,952,450 describes the use of crosslinked polycarbonate for improving of ductility of the polylactic acid. The crosslinked polycarbonate was synthesized via polycondensation of diol, polyhydric alcohol and carbonic acid diesters.
European patent EP2060606 describes a plant sourced resin composition with improved heat resistance where PLA was used as a bio-based component in blend with polycarbonate.
Japanese patent JP2009051989 introduces a polycarbonate/poly(lactide) blend reinforced by silica up to 5% by weight.
There were a number of studies done in the field of PC/PLA blends and their compartibilization. Wang et al., Polymer Engineering and Science 53(6), 1171-1180 (2013) studied blends of PLA and PC where weight average molecular weight of PC was in range of 35000-58000. PC/PLA blends were processed at maximum temperature of 220° C. Researchers found significant drop in impact strength of PLA/PC blends due to poor interphase bonding. In this study the heat deflection of the blends was also negatively affected. Thus, the PLA/PC blends research is mostly concentrated on improving impact strength and heat resistance of the blend via phase modification. Y. Wang et al., Journal of Applied Polymer Science 125(2), 402-412 (2012) used poly(butylene succinate-co-lactate) (PBSL) and epoxy (EP) as compatibilizers in the presence of tetrabutylammonium bromide (TBAB) as a catalyst in a PC/PLA blend processed at 220° C. Researchers reported a moderate increase in impact strength in ternary compositions containing up to 20% PBSL and small increase in heat deflection temperature in blends with 10% of epoxy. Nevertheless, these good properties never converged in one blend composition. An attempt to improve mechanical and especially elongational properties of PLA/PC blends was made by V. T. Phuong et al., Polymer 55, 4498-4513 (2014). Tetrabutylammonium tetraphenylborate (TBATPB) and triacetin were used in a reactive compatibilization of PLA/PC blends at two temperatures of 210° C. and 230° C. This approach allowed significant increasing of elongational properties of the blend. Kazuhiro Hashima et al., Polymer 51, 3934-3939 (2010) investigated PLA/PC blends toughened by blending with hydrogenated styrene-butadiene-styrene block copolymer (SEBS) with the aid of reactive compatibilizer, poly(ethylene-co-glycidyl methacrylate) (EGMA). The processing temperature was 240° C. They were able to manufacture the blend containing 20% of modifiers having high impact strength in the range of 60 kJ/m2 and good elongational properties. This achievement came at expense of lower mechanical properties and reduced HDT. Lee et al., Polymer Degradation and Stability 96, 553-560, (2011) studied PC/PLA blends compatibilized by poly(styrene-co-acrylonitrile)-g-maleic anhydride (SAN-g-MAH), poly(ethylene-co-octene) rubber-maleic anhydride (EOR-MAH) and poly(ethylene-co-glycidyl methacrylate) (EGMA). The molecular weight of PC was 38700 and processing temperature was in range of 240-260° C. It was found that PC/PLA blends compatibilized by EGMA showed worst properties and lowest resistance to degradation.
One of the challenges in processing of PLA/PC blends is an elevated temperature needed for processing the blend. Since PLA is prone to degradation at these temperatures, this represents significant hurdle to successful PLA/PC blending. Relatively recently these limitations were overcome by using of chain extenders which allowed blend processing at higher temperatures without risk of degradation. Chain extenders were used in PLA/PC blends with some success. Y. Srithep et al., Journal of Polymer Engineering 34(7), 665-672 (2014) used an epoxy-functional styrene acrylic copolymer as a chain extender in PLA/PC blends where PC had a weight average molecular weight of 39000. The processing was done at 240° C. Researchers found a favorable effect of chain extender on heat resistance of the blend while having little to no effect on other mechanical properties of the blend. It was attributed to improving of interfacial adhesion in the blend as a side effect of chain extender reaction with blend components.
Among all other things durability of PC/PLA blends remains one of the priorities for successful blending. Harris and Lee, Journal of Applied Polymer Science 128, 2136-2144 (2013) tested commercially available PC/PLA blends and found that all blends exhibited significant loss of mechanical properties after 5 days at 70° C. and 90% of relative humidity. Those blends showed extreme degradation after 14 days of conditioning which resulted in significant loss of mechanical properties and integrity of the samples. Therefore, enhancing the durability of the PC/PLA blends remains a priority.
Overall, while there was significant progress in PLA/PC blend modification, no modification provides balanced mechanical and thermal properties in the blend which might be expected from its components.
There is a need for a PLA/PC composition having excellent mechanical properties as well as excellent heat resistance for durable applications.